An induction furnace is an electrical furnace in which the heat is applied by radio frequency (RF) induction to a conductive sample. FIG. 1 illustrates an induction furnace as used in the prior art. The sample 105 is placed in a non-conductive crucible 101, that is elevated on a pedestal 109 into a combustion tube 101 that has been purged with oxygen. Electrical alternating current energy is applied to the induction coil 107, which in turn induces high currents in the conductive sample. These high currents heat the sample to the point of melting, and some components of the metal may combust. The advantage of the induction furnace is that it is offers a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting. In an analytical application, induction furnaces can be used to melt various types of metals including iron, steel, copper, aluminum, as well as precious metals. One major drawback to induction furnace usage is that the RF at which a sample is heated is not always optimal for a particular application.
In analytical applications, the historical operating frequency has typically remained in the range of about 13 MHz to 20 MHz, depending on the material being melted, and the output power capacity of the furnace. Prior art induction furnaces operate at a frequency that limits the types of samples that could be combusted. This was due to the “skin depth” of the sample which is a measure of the distance RF energy can penetrate beneath the surface of a conductor. For the same conductivity, higher frequency emission has a shallower skin depth that penetrates to a lesser depth into the sample, while lower frequencies can penetrate deeper into thicker samples.
FIG. 2 is a flow chart diagram illustrating the prior art process used in combusting materials using an induction furnace operating at 18 MHz. The process 200 of combusting metals in the induction furnace for analytical applications includes using a combustion accelerator that is inserted into the crucible with the metal sample. The accelerator plays an important role in proper combustion of the sample by an induction furnace. The purity and consistency of the accelerator is very important as it is typically low in both carbon and sulfur content. The role of the accelerator is to couple RF energy into the accelerator material, causing it to melt, which in turn couples thermal energy into the sample. If the sample reaches a critical temperature it will melt and evenly cover the bottom of the crucible, allowing for complete oxidation of any carbon or sulfur in the sample. Typically, one gram (1 g) of accelerator is used with each sample.
Initially in step 201, an accelerator is combusted (without a sample) and in step 202 the instrument determines the amount of analyte (such as carbon or sulfur) present in the accelerator. In step 203, the accelerator is then combined with the sample and is combusted in the induction furnace and analyzed in step 204. In step 205, a determination must be made if the sample was completely combusted. Preferably, the result should be a uniformly molten sample. In step 207, the amount of analyte in the accelerator can be mathematically subtracted from the results. In the event that the burn was not complete and/or uniform, a new sample must be combusted again which can be time consuming and expensive depending on the type of samples involved.
FIG. 3 is an illustration showing combusted samples using the processes described in FIG. 2. Samples 301, 303, 305 are of one type of material such as copper with each placed in its own crucible. The illustrations clearly show only partial burns of the material samples 301, 305 which are inconsistent between each of the samples. Similarly, samples 307, 309, 311 are another type of sample material such as nickel. These samples also show inconsistent and incomplete burns 311 between each of the samples that would require a new sample to be again combusted until an acceptable analysis is obtained.
FIG. 4 is a schematic illustrating an Colpitts induction oscillator circuit used in the induction furnace operating in a range between 13-20 MHz as shown in FIG. 1. In operation, electrical RF energy is coupled in through capacitor 401 to drive the circuit. The series combination of capacitors 403 and 407 form a resonant network with induction coil 405. The ratio of capacitors 403 and 407 set the amplitude of the feedback signal that is coupled through capacitor 409 to the drive circuit. In use, as the oscillator frequency drops, the inductive reactance of the coil 405 becomes substantially a low value that results in high currents which are on the order of 100 Amps. Consequently, the overall value of capacitance must increase as well as the physical size of the capacitors in order to handle these very high currents at low frequencies. Since capacitors 403 and 407 form a set ratio, capacitor 407 must be even larger in value and physical size.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.